1. Field of the Invention
The present invention relates to methods in general, and in particular to industrial scale methods, of preparing .alpha.-L-aspartyl-L-phenylalanine methyl ester hydrochloride (hereinafter referred to as ".alpha.-APM.HCl").
2. Discussion of the Background
.alpha.-APM is a dipeptide sweetener having a sweetness of about 200 times that of sucrose (cane sugar). Because of its extremely good sweetening properties and its low caloric value, it has been used significantly as a diet sweetener. It is expected that the wordwide demand for .alpha.-APM will exceed 10,000 tons before 1995.
.alpha.-APM.HCl may be prepared by a method in which water is added to a reaction mixture (either in the form of a solution or a suspension) of an organic solvent containing N-formyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as "F-.alpha.-APM"). The resultant mixture is heated, extracted with water and the resultant two layers are separated. N-formyl-.alpha.-aspartyl-L-phenylalanine methyl ester which remains dissolved in the separated aqueous layer is then deformylated and the .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as ".alpha.-APM") formed is crystallized out as its hydrochloride salt, and isolated.
Previous methods of producing .alpha.-APM.HCl include:
(i) a method of producing .alpha.-APM as its hydrohalide salt after deprotecting an N-protected-.alpha.-L-aspartyl-L-phenylalanine methyl ester which is formed by a condensation of N-protected-L-aspartic anhydride and L-phenylalanine methyl ester (hereinafter referred to as "L-Phe-OMe" or "PM"); and
(ii) a method of producing .alpha.-APM as its hydrohalide salt after deprotecting an N-protected-.alpha.-L-aspartyl-L-phenylalanine which is formed by condensation of an N-protected-L-aspartic acid anhydride and L-phenylalanine (hereinafter referred to as "L-Phe") followed by methyl-esterifying the deprotected product.
One example of method (i) is described in U.S. Pat. No. 3,786,039. The condensation reaction is carried out in an organic solvent, followed by evaporative removal of the organic solvent, isolation of the resulting F-.alpha.-APM or its PM salt as a solid, and finally deprotecting the formulated product to yield o-APM.HCl. This method, however, requires the isolation of the intermediate F-.alpha.-APM. Due to the complications and additional required steps for isolating this intermediate, this method is not suitable for industrial application.
One example of method (ii) is described in U.S. Pat. No. 3,933,781. N-formyl-.alpha.-L-aspartyl-L-phenylalanine (hereinafter referred to as "F-.alpha.-AP") is formed by condensing N-formyl-L-aspartic acid anhydride and L-Phe in glacial acetic acid, followed by removal of the formyl group from the product to form .alpha.-L-aspartyl-L-phenylanine (hereinafter referred to as ".alpha.-AP"), isolation of the .alpha.-AP intermediate and finally esterification of the isolated intermediate to obtain .alpha.-APM.HCl. As with the other previously described method, this method also suffers the disadvantaqe of requiring isolation of an intermediate compound.
Similarly, U.S. Pat. No. 4,173,562 illustrates a method of obtaining .alpha.-APM.HCl by esterifying isolated F-.alpha.-AP or .alpha.-AP with hydrochloric acid and methanol.
Japanese Patent Application Laid-Open No. 61-143397 illustrates a method of preparing F-.alpha.-AP by condensing N-formyl-L-aspartic acid anhydride and L-Phe in water. In accordance with the method, it is also necessary to isolate F-.alpha.-AP and to deprotect and esterify it to obtain .alpha.-APM.HCl. Therefore, the method is also defective for industrial use.
In all of the above methods, F-.alpha.-AP or .alpha.-AP must be isolated during the procedure. In view of the complications and additional required steps for isolation of these intermediates, these methods are not suitable industrial methods. There is accordingly a need for methods not suffering the above disadvantages.